Fix default value for non-final module type (#1392)

This fixes an issue where the `module` type produces the wrong default
value.

Closes #1391

pkl-core/src/main/java/org/pkl/core/ast/MemberNode.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -16,6 +16,7 @@
 package org.pkl.core.ast;
 
 import com.oracle.truffle.api.frame.FrameDescriptor;
+import com.oracle.truffle.api.frame.VirtualFrame;
 import com.oracle.truffle.api.source.SourceSection;
 import org.pkl.core.ast.member.DefaultPropertyBodyNode;
 import org.pkl.core.runtime.VmExceptionBuilder;
@@ -46,7 +47,8 @@ public abstract class MemberNode extends PklRootNode {
     return new VmExceptionBuilder().withSourceSection(getHeaderSection());
   }
 
-  public boolean isUndefined() {
+  public boolean isUndefined(VirtualFrame frame) {
-    return bodyNode instanceof DefaultPropertyBodyNode propBodyNode && propBodyNode.isUndefined();
+    return bodyNode instanceof DefaultPropertyBodyNode propBodyNode
+        && propBodyNode.isUndefined(frame);
   }
 }

pkl-core/src/main/java/org/pkl/core/ast/expression/member/InferParentWithinMethodNode.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -67,7 +67,7 @@ public final class InferParentWithinMethodNode extends ExpressionNode {
 
     var returnTypeDefaultValue =
         returnTypeNode.createDefaultValue(
-            language, method.getHeaderSection(), method.getQualifiedName());
+            frame, language, method.getHeaderSection(), method.getQualifiedName());
     if (returnTypeDefaultValue != null) {
       inferredParent = returnTypeDefaultValue;
       ownerNode = null;

pkl-core/src/main/java/org/pkl/core/ast/expression/member/InferParentWithinObjectMethodNode.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -75,7 +75,7 @@ public final class InferParentWithinObjectMethodNode extends ExpressionNode {
 
     Object defaultReturnTypeValue =
         returnTypeNode.createDefaultValue(
-            language, member.getHeaderSection(), member.getQualifiedName());
+            frame, language, member.getHeaderSection(), member.getQualifiedName());
     if (defaultReturnTypeValue != null) {
       inferredParent = defaultReturnTypeValue;
       ownerNode = null;

pkl-core/src/main/java/org/pkl/core/ast/expression/member/InferParentWithinPropertyNode.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -18,6 +18,7 @@ package org.pkl.core.ast.expression.member;
 import com.oracle.truffle.api.CompilerDirectives;
 import com.oracle.truffle.api.dsl.NodeChild;
 import com.oracle.truffle.api.dsl.Specialization;
+import com.oracle.truffle.api.frame.VirtualFrame;
 import com.oracle.truffle.api.source.SourceSection;
 import org.pkl.core.ast.ExpressionNode;
 import org.pkl.core.runtime.*;
@@ -35,9 +36,9 @@ public abstract class InferParentWithinPropertyNode extends ExpressionNode {
   }
 
   @Specialization(guards = "!owner.isPrototype()")
-  protected Object evalTypedObject(VmTyped owner) {
+  protected Object evalTypedObject(VirtualFrame frame, VmTyped owner) {
     if (isLocalProperty) {
-      return getLocalPropertyDefaultValue(owner);
+      return getLocalPropertyDefaultValue(frame, owner);
     }
 
     try {
@@ -51,7 +52,7 @@ public abstract class InferParentWithinPropertyNode extends ExpressionNode {
   }
 
   @Specialization(guards = "owner.isPrototype()")
-  protected Object evalPrototype(VmTyped owner) {
+  protected Object evalPrototype(VirtualFrame frame, VmTyped owner) {
     var property =
         isLocalProperty
             ?
@@ -66,7 +67,7 @@ public abstract class InferParentWithinPropertyNode extends ExpressionNode {
     var typeNode = property.getTypeNode();
     if (typeNode == null || typeNode.isUnknownType()) return VmDynamic.empty();
 
-    var result = typeNode.getDefaultValue();
+    var result = typeNode.getDefaultValue(frame);
     if (result != null) return result;
 
     // no default exists for this property type
@@ -76,18 +77,18 @@ public abstract class InferParentWithinPropertyNode extends ExpressionNode {
   }
 
   @Specialization
-  protected Object eval(@SuppressWarnings("unused") VmDynamic owner) {
+  protected Object eval(VirtualFrame frame, @SuppressWarnings("unused") VmDynamic owner) {
     if (isLocalProperty) {
-      return getLocalPropertyDefaultValue(owner);
+      return getLocalPropertyDefaultValue(frame, owner);
     }
 
     return VmDynamic.empty();
   }
 
   @Specialization
-  protected Object eval(@SuppressWarnings("unused") VmListing owner) {
+  protected Object eval(VirtualFrame frame, @SuppressWarnings("unused") VmListing owner) {
     if (isLocalProperty) {
-      return getLocalPropertyDefaultValue(owner);
+      return getLocalPropertyDefaultValue(frame, owner);
     }
 
     assert ownPropertyName == Identifier.DEFAULT;
@@ -96,9 +97,9 @@ public abstract class InferParentWithinPropertyNode extends ExpressionNode {
   }
 
   @Specialization
-  protected Object eval(@SuppressWarnings("unused") VmMapping owner) {
+  protected Object eval(VirtualFrame frame, @SuppressWarnings("unused") VmMapping owner) {
     if (isLocalProperty) {
-      return getLocalPropertyDefaultValue(owner);
+      return getLocalPropertyDefaultValue(frame, owner);
     }
 
     assert ownPropertyName == Identifier.DEFAULT;
@@ -106,13 +107,13 @@ public abstract class InferParentWithinPropertyNode extends ExpressionNode {
     return VmUtils.readMember(BaseModule.getMappingClass().getPrototype(), ownPropertyName);
   }
 
-  private Object getLocalPropertyDefaultValue(VmObjectLike owner) {
+  private Object getLocalPropertyDefaultValue(VirtualFrame frame, VmObjectLike owner) {
     assert isLocalProperty;
 
     var member = owner.getMember(ownPropertyName);
     assert member != null;
 
-    var defaultValue = member.getLocalPropertyDefaultValue();
+    var defaultValue = member.getLocalPropertyDefaultValue(frame);
     if (defaultValue != null) return defaultValue;
 
     // no default exists for this property type

pkl-core/src/main/java/org/pkl/core/ast/member/DefaultPropertyBodyNode.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -38,14 +38,14 @@ public final class DefaultPropertyBodyNode extends ExpressionNode {
     this.typeNode = typeNode;
   }
 
-  public boolean isUndefined() {
+  public boolean isUndefined(VirtualFrame frame) {
-    return typeNode == null || typeNode.getDefaultValue() == null;
+    return typeNode == null || typeNode.getDefaultValue(frame) == null;
   }
 
   @Override
   public Object executeGeneric(VirtualFrame frame) {
     if (typeNode != null) {
-      var defaultValue = typeNode.getDefaultValue();
+      var defaultValue = typeNode.getDefaultValue(frame);
       if (defaultValue != null) return defaultValue;
     }
 

pkl-core/src/main/java/org/pkl/core/ast/member/LocalTypedPropertyNode.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -45,11 +45,11 @@ public final class LocalTypedPropertyNode extends RegularMemberNode {
     this.unresolvedTypeNode = unresolvedTypeNode;
   }
 
-  public @Nullable Object getDefaultValue() {
+  public @Nullable Object getDefaultValue(VirtualFrame frame) {
     if (!defaultValueInitialized) {
       defaultValue =
           typeNode.createDefaultValue(
-              language, member.getHeaderSection(), member.getQualifiedName());
+              frame, language, member.getHeaderSection(), member.getQualifiedName());
       defaultValueInitialized = true;
     }
     return defaultValue;

pkl-core/src/main/java/org/pkl/core/ast/member/ObjectMember.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -17,6 +17,7 @@ package org.pkl.core.ast.member;
 
 import com.oracle.truffle.api.CompilerDirectives.CompilationFinal;
 import com.oracle.truffle.api.RootCallTarget;
+import com.oracle.truffle.api.frame.VirtualFrame;
 import com.oracle.truffle.api.source.SourceSection;
 import org.pkl.core.ast.ConstantNode;
 import org.pkl.core.ast.MemberNode;
@@ -102,14 +103,14 @@ public final class ObjectMember extends Member {
     return callTarget;
   }
 
-  public boolean isUndefined() {
+  public boolean isUndefined(VirtualFrame frame) {
-    return getMemberNode() != null && getMemberNode().isUndefined();
+    return getMemberNode() != null && getMemberNode().isUndefined(frame);
   }
 
-  public @Nullable Object getLocalPropertyDefaultValue() {
+  public @Nullable Object getLocalPropertyDefaultValue(VirtualFrame frame) {
     assert isProp() && isLocal();
     return getMemberNode() instanceof LocalTypedPropertyNode propertyNode
-        ? propertyNode.getDefaultValue()
+        ? propertyNode.getDefaultValue(frame)
         : VmDynamic.empty();
   }
 

pkl-core/src/main/java/org/pkl/core/ast/member/PropertyTypeNode.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -64,11 +64,11 @@ public final class PropertyTypeNode extends PklRootNode {
     return typeNode.execute(frame, frame.getArguments()[2]);
   }
 
-  public @Nullable Object getDefaultValue() {
+  public @Nullable Object getDefaultValue(VirtualFrame frame) {
     if (!defaultValueInitialized) {
       defaultValue =
           typeNode.createDefaultValue(
-              VmLanguage.get(this), getSourceSection(), qualifiedPropertyName);
+              frame, VmLanguage.get(this), getSourceSection(), qualifiedPropertyName);
       defaultValueInitialized = true;
     }
     return defaultValue;

pkl-core/src/main/java/org/pkl/core/ast/type/GetParentForTypeNode.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -26,6 +26,7 @@ import org.pkl.core.util.LateInit;
 /** Resolves `<type>` to the type's default value in `new <type> { ... }`. */
 public final class GetParentForTypeNode extends ExpressionNode {
   @Child private UnresolvedTypeNode unresolvedTypeNode;
+  @Child private TypeNode typeNode;
   private final String qualifiedName;
 
   @CompilationFinal @LateInit Object defaultValue;
@@ -37,14 +38,24 @@ public final class GetParentForTypeNode extends ExpressionNode {
     this.qualifiedName = qualifiedName;
   }
 
+  private TypeNode getTypeNode(VirtualFrame frame) {
+    if (typeNode == null) {
+      CompilerDirectives.transferToInterpreterAndInvalidate();
+      typeNode = unresolvedTypeNode.execute(frame);
+      adoptChildren();
+    }
+    return typeNode;
+  }
+
   @Override
   public Object executeGeneric(VirtualFrame frame) {
     if (defaultValue != null) return defaultValue;
 
     CompilerDirectives.transferToInterpreterAndInvalidate();
 
-    var typeNode = unresolvedTypeNode.execute(frame);
+    var typeNode = getTypeNode(frame);
-    defaultValue = typeNode.createDefaultValue(VmLanguage.get(this), sourceSection, qualifiedName);
+    defaultValue =
+        typeNode.createDefaultValue(frame, VmLanguage.get(this), sourceSection, qualifiedName);
 
     if (defaultValue != null) {
       unresolvedTypeNode = null;

pkl-core/src/main/java/org/pkl/core/ast/type/TypeNode.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -116,6 +116,7 @@ public abstract class TypeNode extends PklNode {
 
   // method arguments are used when default value contains a root node
   public @Nullable Object createDefaultValue(
+      VirtualFrame frame,
       VmLanguage language,
       // header section of the property or method that carries the type annotation
       SourceSection headerSection,
@@ -473,6 +474,16 @@ public abstract class TypeNode extends PklNode {
     protected boolean acceptTypeNode(TypeNodeConsumer consumer) {
       return consumer.accept(this);
     }
+
+    @Override
+    public @Nullable Object createDefaultValue(
+        VirtualFrame frame,
+        VmLanguage language,
+        SourceSection headerSection,
+        String qualifiedName) {
+      var moduleClass = ((VmTyped) getModuleNode.executeGeneric(frame)).getVmClass();
+      return TypeNode.createDefaultValue(moduleClass);
+    }
   }
 
   public static final class StringLiteralTypeNode extends ObjectSlotTypeNode {
@@ -504,7 +515,10 @@ public abstract class TypeNode extends PklNode {
 
     @Override
     public Object createDefaultValue(
-        VmLanguage language, SourceSection headerSection, String qualifiedName) {
+        VirtualFrame frame,
+        VmLanguage language,
+        SourceSection headerSection,
+        String qualifiedName) {
 
       return literal;
     }
@@ -572,7 +586,10 @@ public abstract class TypeNode extends PklNode {
 
     @Override
     public Object createDefaultValue(
-        VmLanguage language, SourceSection headerSection, String qualifiedName) {
+        VirtualFrame frame,
+        VmLanguage language,
+        SourceSection headerSection,
+        String qualifiedName) {
 
       return VmDynamic.empty();
     }
@@ -622,7 +639,10 @@ public abstract class TypeNode extends PklNode {
 
     @Override
     public @Nullable Object createDefaultValue(
-        VmLanguage language, SourceSection headerSection, String qualifiedName) {
+        VirtualFrame frame,
+        VmLanguage language,
+        SourceSection headerSection,
+        String qualifiedName) {
 
       return TypeNode.createDefaultValue(clazz);
     }
@@ -691,7 +711,10 @@ public abstract class TypeNode extends PklNode {
 
     @Override
     public @Nullable Object createDefaultValue(
-        VmLanguage language, SourceSection headerSection, String qualifiedName) {
+        VirtualFrame frame,
+        VmLanguage language,
+        SourceSection headerSection,
+        String qualifiedName) {
 
       return TypeNode.createDefaultValue(clazz);
     }
@@ -734,10 +757,13 @@ public abstract class TypeNode extends PklNode {
 
     @Override
     public @Nullable Object createDefaultValue(
-        VmLanguage language, SourceSection headerSection, String qualifiedName) {
+        VirtualFrame frame,
+        VmLanguage language,
+        SourceSection headerSection,
+        String qualifiedName) {
 
       return VmNull.withDefault(
-          elementTypeNode.createDefaultValue(language, headerSection, qualifiedName));
+          elementTypeNode.createDefaultValue(frame, language, headerSection, qualifiedName));
     }
 
     @Override
@@ -817,12 +843,15 @@ public abstract class TypeNode extends PklNode {
 
     @Override
     public @Nullable Object createDefaultValue(
-        VmLanguage language, SourceSection headerSection, String qualifiedName) {
+        VirtualFrame frame,
+        VmLanguage language,
+        SourceSection headerSection,
+        String qualifiedName) {
 
       return defaultIndex == -1
           ? null
           : elementTypeNodes[defaultIndex].createDefaultValue(
-              language, headerSection, qualifiedName);
+              frame, language, headerSection, qualifiedName);
     }
 
     @Override
@@ -1021,9 +1050,11 @@ public abstract class TypeNode extends PklNode {
     }
 
     @Override
-    @TruffleBoundary
     public @Nullable Object createDefaultValue(
-        VmLanguage language, SourceSection headerSection, String qualifiedName) {
+        VirtualFrame frame,
+        VmLanguage language,
+        SourceSection headerSection,
+        String qualifiedName) {
 
       return unionDefault;
     }
@@ -1063,7 +1094,10 @@ public abstract class TypeNode extends PklNode {
 
     @Override
     public Object createDefaultValue(
-        VmLanguage language, SourceSection headerSection, String qualifiedName) {
+        VirtualFrame frame,
+        VmLanguage language,
+        SourceSection headerSection,
+        String qualifiedName) {
 
       return VmList.EMPTY;
     }
@@ -1152,7 +1186,10 @@ public abstract class TypeNode extends PklNode {
 
     @Override
     public Object createDefaultValue(
-        VmLanguage language, SourceSection headerSection, String qualifiedName) {
+        VirtualFrame frame,
+        VmLanguage language,
+        SourceSection headerSection,
+        String qualifiedName) {
 
       return VmList.EMPTY;
     }
@@ -1240,7 +1277,10 @@ public abstract class TypeNode extends PklNode {
 
     @Override
     public final Object createDefaultValue(
-        VmLanguage language, SourceSection headerSection, String qualifiedName) {
+        VirtualFrame frame,
+        VmLanguage language,
+        SourceSection headerSection,
+        String qualifiedName) {
 
       return VmSet.EMPTY;
     }
@@ -1332,7 +1372,10 @@ public abstract class TypeNode extends PklNode {
 
     @Override
     public Object createDefaultValue(
-        VmLanguage language, SourceSection headerSection, String qualifiedName) {
+        VirtualFrame frame,
+        VmLanguage language,
+        SourceSection headerSection,
+        String qualifiedName) {
 
       return VmMap.EMPTY;
     }
@@ -1586,45 +1629,48 @@ public abstract class TypeNode extends PklNode {
       return valueTypeCastNode;
     }
 
-    // either (if defaultMemberValue != null):
-    // x: Listing<Foo> // = new Listing {
-    //   default = name -> new Foo {}
-    // }
-    // or (if defaultMemberValue == null):
-    // x: Listing<Int> // = new Listing {
-    //   default = Undefined()
-    // }
-    @Override
     @TruffleBoundary
-    public final Object createDefaultValue(
+    private Object newEmptyListingOrMapping() {
-        VmLanguage language, SourceSection headerSection, String qualifiedName) {
+      if (isListing()) {
-
+        return new VmListing(
-      if (valueTypeNode instanceof UnknownTypeNode) {
-        if (isListing()) {
-          return new VmListing(
-              VmUtils.createEmptyMaterializedFrame(),
-              BaseModule.getListingClass().getPrototype(),
-              EconomicMaps.create(),
-              0);
-        }
-
-        return new VmMapping(
             VmUtils.createEmptyMaterializedFrame(),
-            BaseModule.getMappingClass().getPrototype(),
+            BaseModule.getListingClass().getPrototype(),
-            EconomicMaps.create());
+            EconomicMaps.create(),
+            0);
       }
 
+      return new VmMapping(
+          VmUtils.createEmptyMaterializedFrame(),
+          BaseModule.getMappingClass().getPrototype(),
+          EconomicMaps.create());
+    }
+
+    @TruffleBoundary
+    private Object newEmptyListingOrMapping(ObjectMember defaultMember) {
+      if (isListing()) {
+        return new VmListing(
+            VmUtils.createEmptyMaterializedFrame(),
+            BaseModule.getListingClass().getPrototype(),
+            EconomicMaps.of(Identifier.DEFAULT, defaultMember),
+            0);
+      }
+
+      return new VmMapping(
+          VmUtils.createEmptyMaterializedFrame(),
+          BaseModule.getMappingClass().getPrototype(),
+          EconomicMaps.of(Identifier.DEFAULT, defaultMember));
+    }
+
+    @TruffleBoundary
+    private ObjectMember createDefaultMember(
+        SourceSection headerSection, String qualifiedName, @Nullable Object defaultMemberValue) {
       var defaultMember =
           new ObjectMember(
               headerSection,
               headerSection,
               VmModifier.HIDDEN,
               Identifier.DEFAULT,
-              qualifiedName + ".default");
+              VmUtils.concat(qualifiedName, ".default"));
-
-      var defaultMemberValue =
-          valueTypeNode.createDefaultValue(language, headerSection, qualifiedName);
-
       if (defaultMemberValue == null) {
         defaultMember.initMemberNode(
             new UntypedObjectMemberNode(
@@ -1645,23 +1691,38 @@ public abstract class TypeNode extends PklNode {
                     language,
                     new FrameDescriptor(),
                     headerSection,
-                    defaultMember.getQualifiedName() + ".<function>",
+                    VmUtils.concat(defaultMember.getQualifiedName(), ".<function>"),
                     new ConstantValueNode(defaultMemberValue)),
                 null));
       }
+      return defaultMember;
+    }
 
-      if (isListing()) {
+    // either (if defaultMemberValue != null):
-        return new VmListing(
+    // x: Listing<Foo> // = new Listing {
-            VmUtils.createEmptyMaterializedFrame(),
+    //   default = name -> new Foo {}
-            BaseModule.getListingClass().getPrototype(),
+    // }
-            EconomicMaps.of(Identifier.DEFAULT, defaultMember),
+    // or (if defaultMemberValue == null):
-            0);
+    // x: Listing<Int> // = new Listing {
+    //   default = Undefined()
+    // }
+    @Override
+    public final Object createDefaultValue(
+        VirtualFrame frame,
+        VmLanguage language,
+        SourceSection headerSection,
+        String qualifiedName) {
+
+      if (valueTypeNode instanceof UnknownTypeNode) {
+        return newEmptyListingOrMapping();
       }
 
-      return new VmMapping(
+      var defaultMemberValue =
-          VmUtils.createEmptyMaterializedFrame(),
+          valueTypeNode.createDefaultValue(frame, language, headerSection, qualifiedName);
-          BaseModule.getMappingClass().getPrototype(),
+
-          EconomicMaps.of(Identifier.DEFAULT, defaultMember));
+      var defaultMember = createDefaultMember(headerSection, qualifiedName, defaultMemberValue);
+
+      return newEmptyListingOrMapping(defaultMember);
     }
 
     protected void doEagerCheck(VirtualFrame frame, VmObject object) {
@@ -2030,7 +2091,10 @@ public abstract class TypeNode extends PklNode {
 
     @Override
     public @Nullable Object createDefaultValue(
-        VmLanguage language, SourceSection headerSection, String qualifiedName) {
+        VirtualFrame frame,
+        VmLanguage language,
+        SourceSection headerSection,
+        String qualifiedName) {
 
       CompilerDirectives.transferToInterpreter();
       throw exceptionBuilder()
@@ -2441,34 +2505,41 @@ public abstract class TypeNode extends PklNode {
       }
     }
 
-    @Override
     @TruffleBoundary
+    private VmFunction newMixin(VmLanguage language, String qualifiedName) {
+      //noinspection ConstantConditions
+      return new VmFunction(
+          VmUtils.createEmptyMaterializedFrame(),
+          // Assumption: don't need to set the correct `thisValue`
+          // because it is guaranteed to be never accessed.
+          null,
+          1,
+          new IdentityMixinNode(
+              language,
+              new FrameDescriptor(),
+              getSourceSection(),
+              qualifiedName,
+              typeArgumentNodes.length == 1
+                  ?
+                  // shouldn't need to deepCopy() this node because it isn't used as @Child
+                  // anywhere else
+                  typeArgumentNodes[0]
+                  : null),
+          null);
+    }
+
+    @Override
     public @Nullable Object createDefaultValue(
-        VmLanguage language, SourceSection headerSection, String qualifiedName) {
+        VirtualFrame frame,
+        VmLanguage language,
+        SourceSection headerSection,
+        String qualifiedName) {
 
       if (typeAlias == BaseModule.getMixinTypeAlias()) {
-        //noinspection ConstantConditions
+        return newMixin(language, qualifiedName);
-        return new VmFunction(
-            VmUtils.createEmptyMaterializedFrame(),
-            // Assumption: don't need to set the correct `thisValue`
-            // because it is guaranteed to be never accessed.
-            null,
-            1,
-            new IdentityMixinNode(
-                language,
-                new FrameDescriptor(),
-                getSourceSection(),
-                qualifiedName,
-                typeArgumentNodes.length == 1
-                    ?
-                    // shouldn't need to deepCopy() this node because it isn't used as @Child
-                    // anywhere else
-                    typeArgumentNodes[0]
-                    : null),
-            null);
       }
 
-      return aliasedTypeNode.createDefaultValue(language, headerSection, qualifiedName);
+      return aliasedTypeNode.createDefaultValue(frame, language, headerSection, qualifiedName);
     }
 
     @Override
@@ -2593,9 +2664,12 @@ public abstract class TypeNode extends PklNode {
 
     @Override
     public @Nullable Object createDefaultValue(
-        VmLanguage language, SourceSection headerSection, String qualifiedName) {
+        VirtualFrame frame,
+        VmLanguage language,
+        SourceSection headerSection,
+        String qualifiedName) {
 
-      return childNode.createDefaultValue(language, headerSection, qualifiedName);
+      return childNode.createDefaultValue(frame, language, headerSection, qualifiedName);
     }
 
     public SourceSection getBaseTypeSection() {

pkl-core/src/main/java/org/pkl/core/runtime/MirrorFactories.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -184,7 +184,10 @@ public final class MirrorFactories {
             property ->
                 property.getProperty().isAbstract()
                         || property.getProperty().isExternal()
-                        || property.getProperty().getInitializer().isUndefined()
+                        || property
+                            .getProperty()
+                            .getInitializer()
+                            .isUndefined(VmUtils.createEmptyMaterializedFrame())
                     ? VmNull.withoutDefault()
                     :
                     // get default from prototype because it's cached there

pkl-core/src/main/java/org/pkl/core/runtime/VmUtils.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -909,4 +909,9 @@ public final class VmUtils {
     return frame.getArguments().length != 4
         || frame.getArguments()[3] != VmUtils.SKIP_TYPECHECK_MARKER;
   }
+
+  @TruffleBoundary
+  public static String concat(String str1, String str2) {
+    return str1 + str2;
+  }
 }

pkl-core/src/test/files/LanguageSnippetTests/input/types/moduleType2.pkl

@@ -0,0 +1,20 @@
+open module Foo
+
+prop: String
+
+myself: module?
+
+class Bar extends module {
+  prop2: Int
+}
+
+output {
+  value = new Bar {
+    prop = "hi"
+    prop2 = 15
+    myself {
+      prop = "hi again"
+      prop2 = 15
+    }
+  }
+}

pkl-core/src/test/files/LanguageSnippetTests/output/types/moduleType2.pcf

@@ -0,0 +1,7 @@
+prop = "hi"
+myself {
+  prop = "hi again"
+  myself = null
+  prop2 = 15
+}
+prop2 = 15